A member of the long-necked sauropod group of dinosaurs which includes Diplodocus and Brachiosaurus, Brontomerus may have used its powerful thighs as a weapon to kick predators, or to help travel over rough, hilly terrain. Brontomerus lived about 110 million years ago, during the Early Cretaceous Period, and probably had to contend with fierce raptors such as Deinonychus and Utahraptor.

The fossilized bones of two specimens  an adult and a juvenile  of Brontomerus mcintoshi were rescued by researchers from the Sam Noble Museum from a quarry in eastern Utah that had been looted and damaged, possibly by commercial fossil hunters. Paleontologists speculate that the larger specimen is the mother of the younger and would have weighed around 6 tons (about the size of a large elephant) and measured 14 meters in length.

The authors classified the new genus based on an incomplete skeleton, now held at the Sam Noble Museum, including bones from the shoulder, hip, ribs, vertebrae and some unidentifiable fragments. They used the bones to identify Brontomerus unique features, primarily the shape of the ilium (hip bone), which, in the case of Brontomerus, is unusually large in comparison to that of similar dinosaurs. The wide, blade-shaped bone projects forward ahead of the hip socket, providing a proportionally massive area for the attachment of muscles. The shape of the bone indicates that the animal would likely have had the largest leg muscles of any dinosaur in the sauropod family. This is reflected in the name Brontomerus, which literally means thunder thighs. The dinosaurs species name, mcintoshi, was chosen in honor of John Jack McIntosh, a retired physicist at Wesleyan University, Conn., and lifelong avocational paleontologist.

Brontomerus mcintoshi is a charismatic dinosaur and an exciting discovery for us, said Taylor. When we recognized the weird shape of the hip, we wondered what its significance might be, but we concluded that kicking was the most likely. The kick would probably have been used when two males fought over a female, but given that the mechanics were all in place it would be bizarre if it wasn't also used in predator defense.

Other marks on the bones give additional clues to Brontomerus lifestyle and environment. Wedel explained: The shoulder blade of Brontomerus has unusual bumps that probably mark the boundaries of muscle attachments, suggesting that Brontomerus had powerful forelimb muscles as well. Its possible that Brontomerus mcintoshi was more athletic than most other sauropods. It is well established that far from being swamp-bound hippo-like animals, sauropods preferred drier, upland areas; so perhaps Brontomerus lived in rough, hilly terrain and the powerful leg muscles were a sort of dinosaur four-wheel drive.

While Brontomerus unusual hip structure and enormous thigh muscles place it on the list of most extreme dinosaurs, it is also a significant find for another reason. It is one more in a number of finds over the past 20 years that challenge the previously held idea that sauropods began to disappear in the Early Cretaceous period.

Because sauropods were the most abundant dinosaurs found during the Jurassic period and the rarest during the Early Cretaceous, theres long been the perception that sauropods were successful in the Jurassic and were replaced by duckbills and horned dinosaurs in the Cretaceous, explained Wedel. In the past 20 years, however, we are finding more sauropods from the Early Cretaceous period, and the picture is changing. It now seems that sauropods may have been every bit as diverse as they were during the Jurassic, but much less abundant and so much less likely to be found.